(1) Field of the Invention
The present invention generally relates to evaporative fuel control apparatus, and more particularly to an evaporative fuel control apparatus of an internal combustion engine for feeding fuel vapor from a fuel tank to an intake system through a purge passage in which a purge control valve is provided.
(2) Description of the Related Art
Conventionally, in an internal combustion engine, an evaporative fuel control apparatus has been used, which stores fuel vapor from a fuel tank by activated carbon of a canister and feeds the stored fuel vapor from the canister into an intake passage of the intake system of the internal combustion engine. The feeding of fuel vapor into the intake passage is called hereinafter the purging of fuel vapor. Also, there is a known internal combustion engine which has a fuel injection control part for performing a feedback control of an air-fuel ratio of air fuel mixture to control it convergently toward the stoichiometric air-fuel ratio. Conventionally, when the feedback control of the air-fuel ratio is performed, it is considered that a feedback correction factor FAF adjusts suitably the air-fuel ratio, and a description of the feedback correction factor or coefficient FAF is disclosed, for example, in the U.S. Pat. No. 4,841,940 assigned to the same assignee with the present invention, and the disclosure of this patent regarding the term "feedback correction coefficient" is hereby incorporated in the present specification for the sake of clarity.
Also, there is another internal combustion engine having a fuel control part which has been proposed by the same applicant, as disclosed in Japanese Laid-Open Patent Application No.63-55357. In the case of this prior art internal combustion engine, the feedback control of the air-fuel ratio is performed within a first range of the feedback correction factor. When the value of the feedback correction factor is within a given range which is slightly narrower than the first range in which the feedback control of the air-fuel ratio is performed, the amount of fuel vapor purged into the intake passage is corrected to increase gradually. When the value of the feedback correction factor is not within the above given range, the amount of fuel vapor purged is corrected to decrease it gradually, so that effective purging of fuel vapor is carried out to attain appropriate feedback control of the air-fuel ratio.
However, in the case of the above mentioned fuel control part of the internal combustion engine, there is a problem in that the amount of fuel vapor being purged is occasionally excessively adjusted, causing the response of the feedback control of the air-fuel ratio to become worse. This is because the purging amount of fuel vapor is adjusted merely by making a determination as to whether the value of the feedback correction factor is within the given range or not. For example, when the feedback correction factor changes from a value outside a rich-side range between 1.0 and a given high reference level to a value within the rich-side range, the amount of fuel vapor purged, in the case of the prior art apparatus, is unnecessarily adjusted to decrease it. Accordingly, the amount of the correction performed to correct the air-fuel ratio by the feedback control will be excessive so that the air fuel ratio will be adjusted to an excessive level, thus causing the feedback control of the air-fuel ratio in response to the changes in the operating conditions becomes less accurate and slower.